Leeds have been on the trail of the Huesca no 10 since the end of the Spanish season in June after Saiz helped his club finish sixth in the Segunda Division, qualifying for the play-offs.
Talks with Huesca progressed last week but the 26-year-old has since been on holiday in Miami from where the Spaniard will fly back home on Sunday.
The former Atletico Madrid attacker is then due in Leeds on Monday with the playmaker expected to wrap up his move to United for a fee in the region of £3m.
Saiz made 43 appearances for Huesca last season, scoring 12 times.
A host of other Spanish clubs have been interested in him, notably Eibar who were particularly keen to seal his signature.
Leeds sealed their sixth signing of the summer on Thursday when former Newcastle United player Vurnon Anita joined the club on a three-year deal, after which head coach Thomas Christiansen said there would be “a few” more players signed in the “next few days”.